Extracellular Vesicles in Immune System Regulation and Type 1 Diabetes: Cell-to-Cell Communication Mediators, Disease Biomarkers, and Promising Therapeutic Tools

Abstract

Extracellular vesicles (EVs) are generated by cells of origin through complex molecular mechanisms and released into extracellular environment. Hence, the presence of EVs has been described in multiple biological fluids and in most cases their molecular cargo, which includes non-coding RNAs (ncRNA), messenger RNAs (mRNA), and proteins, has been reported to modulate distinct biological processes. EVs release and their molecular cargo have been demonstrated to be altered in multiple diseases, including autoimmune diseases. Notably, numerous evidence showed a relevant crosstalk between immune system and interacting cells through specific EVs release. The crosstalk between insulin-producing pancreatic β cells and immune system through EVs bidirectional trafficking has yet started to be deciphered, thus uncovering an intricate communication network underlying type 1 diabetes (T1D) pathogenesis. EVs can also be found in blood plasma or serum. Indeed, the assessment of circulating EVs cargo has been shown as a promising advance in the detection of reliable biomarkers of disease progression. Of note, multiple studies showed several specific cargo alterations of EVs collected from plasma/serum of subjects affected by autoimmune diseases, including T1D subjects. In this review, we discuss the recent literature reporting evidence of EVs role in autoimmune diseases, specifically focusing on the bidirectional crosstalk between pancreatic β cells and immune system in T1D and highlight the relevant promising role of circulating EVs as disease biomarkers.

Keywords: autoimmunity; exosomes; extracellular vesicles; immune regulation; pancreatic islets; type 1 diabetes.

Figure 1

Exosomes biogenesis and composition. Exosomes biogenesis is based on a process of double invagination of the plasma membrane. The first step leading to the invagination process results into de novo formation of an early-sorting endosome characterized by cell-surface proteins and soluble proteins. Following the continuous exchange (import and export) of specific proteins and molecules from endosomes and mitochondria as well as with Trans Golgi network, the early-sorting endosome can mature into a late-sorting endosome through a second invagination of the plasma membrane leading to the generation of the Multi Vesicular Bodies (MVBs) composed by multiple intraluminal vesicles (ILVs). Then, MVBs can: (i) fuse with lysosomes resulting in the digestion of their ILVs or (ii) fuse with the plasma membrane thus leading to the release of ILVs as exosomes (22, 23). Exosomes expose several membrane proteins, such as tetraspanins (CD9, CD81, CD63) (–28) and MHC molecules, both class I and class II, as well as Annexin and Flotillin. Moreover, exosomes content is highly variable depending on the cell of origin, but usually they contain cytoskeleton proteins (e.g., actin and tubulin), heat shock proteins such as HSP70 and HSP90, proteins associated to metabolism (GAPDH) (–31), as well as specific proteins (e.g., ALIX, TSG101 and RAB proteins). Exosomes also contains different RNA species such as microRNAs, among which the most reported are miR-126-3p and miR-223-3p (–36).

Figure 2

Crosstalk between β cells and immune cells through EVs. In T1D context, β cells and immune cells tightly communicate each other. β cells are subjected to glucotoxic and/or inflammatory stress and can release EVs containing specific miRNAs characterized by a GU-rich sequence (let-7a/b/c, miR-21 and miR-29a) which are transferred to resident Antigen Presenting Cells (APCs), where these miRNAs can bind to endosomal Toll Like Receptor 7 (TLR7) leading to the activation of inflammatory signals (–142). Moreover, under inflammatory stress, β cells secrete and transfer EVs to APCs. Such EVs can contain specific autoantigens such as insulin, IA2, ZNT8, GAD65 and proinsulin (5) thus leading to their transfer to APC which can present these antigens for adaptive immunity activation. Activated APC can also lead to CD4⁺ T cells activation through two different mechanisms: (i) release of EVs containing insulin, IA2, ZNT8, GAD65 and proinsulin autoantigens (4), or (ii) release of EVs exposing MHC-II on their surface through which APCs present autoantigens to CD4⁺ T cells (86, 87), leading to autoreactive T cell activation and subsequent β cell destruction. In T1D context, pancreatic islet-infiltrating T cells secrete a specific subpopulation of EVs carrying miR-142-3p/5p and miR-155 which can be transferred to β cells; these miRNAs cause the upregulation of inflammatory molecules such as CCL2, CCL7 and CXCL10 leading to β cell apoptosis (6).